In vitro activity: AM-1241 is a protean agonist of CB2 based on the different effect observed in various assays (calcium influx, extracellular signal-regulated kinase (ERK) phosphorylatin and cAMP measurement)) and on the switch from neutral antagonism to agonism in the cAMP assay when forskolin concentration is lowered. In [3H]CP 55,940 competition binding assays, AM-1241 displays high affinity at the human CB2 receptor with a Ki value of about 7 nM, whereas its affinity at the human CB1 receptor is more than 80-fold weaker, using membrane preparations from stable HEK and CHO cell lines expressing the recombinant human CB2 and CB1 receptors, respectively.

Kinase Assay: Binding to cannabinoid receptors is tested by using competition-equilibrium binding vs. [3H]CP55,940. AM-1241 is diluted into 25 mM Tris base (pH 7.4)/5 mM MgCl2/1 mM EDTA/0.1% essentially fatty acid-free BSA and transferred to Regisil-treated 96-well plates. [3H]CP55,940 (DuPont_NEN; specific activity 100–180 Ci/ mmol; 1 Ci =37 GBq) is added to a concentration of 0.8 nM. Membranes prepared from rat brain (containing CB1 receptors) or mouse spleen (containing CB2 receptors) are added (≈50 μg of membrane protein per well), plates are incubated at 30 °C for 1 hour, and the contents are filtered over Packard Unifilter GF/B 96-well filters by using a Packard Filtermate 196 cell harvester. Filters are washed with ice-cold 50 mM Tris base/5 mM MgCl2/0.5% BSA and dried. Bound radioactivity is quantitated and corrected for nonspecific binding, and results are normalized between 0% and 100% [3H]CP-55,940 specifically bound. IC50 is determined by nonlinear regression analysis using GraphPad PRISM and transformed to a Ki value. All data are collected in duplicate. IC50 and Ki values are determined from three independent experiments.

Cell Assay: Membrane samples are prepared from HEK cells stably expressing the human CB2 receptors previously generated (Mukherjee et al., 2004), or the CHO cell line that stably expresses the human CB1 receptor. Radioligand binding assays are performed as following. Briefly, the cells are harvested and homogenized using a Polytron for 2 × 10 s bursts in a buffer containing 50 mM Tris-HCl, pH 7.4, 1 mM MgCl2, and 1mM EDTA in the presence of protease inhibitors followed by centrifugation at 45 000 g for 20 minutes. The membrane pellets are washed and frozen at -80 °C in aliquots until use. Saturation binding reactions are performed at 30 °C for 90 minutes using [3H]CP 55,940 (0.01–8 nM) in an assay buffer containing 50 mM Tris-HCl, pH 7.4, 2.5 mM EDTA, 5mM MgCl2, and 0.05% fatty acid free bovine serum albumin (BSA) and the reactions are terminated by rapid vacuum filtration through UniFilter-96 GF/C filter plates and four washes with cold assay buffer. Competition experiments are conducted using 0.5 nM [3H]CP 55,940 in the presence of test compounds (0.1 nM–10 μM). Nonspecific binding is defined by 10 mM unlabeled CP 55,940. KD values from saturation binding assays and Ki values from competition binding assays are determined with one site binding or one site competition curve fitting using the Prism software.

Br J Pharmacol. 2006 Sep;149(2):145-54; Neuroscience. 2003;119(3):747-57.